This invention relates to a magnetic-drive centrifugal pump for delivering a fluid under pressure by an impeller driven by a driving motor through a magnetic coupling, and more particularly to a sealless pump which is easy to assemble and disassemble for its maintenance and inspection and superior in chemical corrosion-resistance.
In a magnetic-drive centrifugal pump, a pump rotor and a driving motor are magnetically coupled by a magnetic coupling to transmit rotating torques therebetween, so that a liquid to be delivered does not leak along a pump shaft without using shaft sealing means. Accordingly, such a pump has been widely used for transferring chemical medicines, petroleum and beverages. In this case, the magnetic coupling is accomplished by arranging a driving magnet concentric to and outside an annular impeller magnet provided in an impeller.
Such a magnetic-drive centrifugal pump has a construction as shown in FIG. 1. The pump mainly comprises a pump shaft 1, and an impeller 2 a rotor 3 rotatably mounted through bearings 5 on the pump shaft 1. One end of the pump shaft 1 is journaled its one end in a hub or boss 16 supported by ribs 15 provided in a fluid inlet 13 in a front casing 11 of a pump casing 10 and the other end is journaled in a center of a rear wall of a rear casing 12 accommodating the rotor 3.
In an outer periphery of the rotor 3 is provided a driven or impeller magnet 6 concentric to the pump shaft 1. About an outer periphery of the rear casing 12 a driving magnet 20 concentric to the impeller magnet 6 is provided in a magnet holder 21. The magnet holder 21 is received in a magnet housing 31 and connected to a driving motor 30. A connection between the front casing 11 and the rear casing 12 is sealed by an O-ring 17. The front casing 11 is provided with an outlet 14 for a fluid in a radial direction of blades of the impeller 2. In this manner a pump casing is formed.
With this arrangement of the pump of the prior art, the bearings for the pump shaft 1 are located on an axis of the impeller 2, so that circumferential speeds of the bearings are relatively low. Accordingly, there are advantages in this arrangement in that relatively small bearings can be employed and life spans of the bearings can be elongated, and that the impeller 2 and the rotor 3 having the impeller magnet 6 can be integrally formed.
However, such a pump of the prior art has been used only in relatively low-torque applications, for example, for fluids having small specific gravities or low viscosities because of limited torque which can be transmitted by the magnetic-drive.
In order to solve this problem, it may be considered to use a large impeller magnet or a large rotor. However, the large rotor tends to make difficult the assembling and disassembling of the pump in manufacture it or maintenance and inspection. Such a difficulty is caused by the fact that a pump shaft for supporting the rotor is supported only by a rear wall of a rear casing when the pump is being assembled or disassembled, and the rear wall of the rear casing is subjected to a great moment. In disassembling of the pump, particularly, a great moment is caused by a slight deflection of the pump shaft when its front end is removed from a hub or boss. As a result, such a great moment often damages the pump shaft or the rear casing. When the pump shaft and the rear casing are made of a ceramic material in order to improve their chemical corrosion-resistance, particularly, these members are likely to be damaged because of the brittleness of the ceramic material. To avoid this, it may be considered to enlarge the a diameter of the pump shaft or thickness of the rear wall of the rear casing. However, such an enlargement of the members does not serve to improve a performance of the pump but only makes the pump bulky.